1. Field of the Invention
The invention relates to a variable frequency oscillator, more particularly to a variable frequency oscillator which can be implemented entirely in a semiconductor chip, which is variable over a broad frequency range and which has relatively low phase noise.
2. Description of the Related Art
Variable frequency oscillators can be classified as tuned oscillators or as relaxation oscillators. Tuned oscillators employ LC-tank circuits or crystals and are known to be operable and stable at high frequencies and are also known to have relatively low phase noise. However, frequency adjustment of conventional tuned oscillators is usually limited to a very narrow range. In addition, conventional tuned oscillators require expensive and bulky external circuitry, such as varactors. Thus, conventional tuned oscillators are seldom used in integrated circuits.
Relaxation oscillators, on the other hand, can be implemented in semiconductor chips as monolithic integrated circuits. Although relaxation oscillators permit frequency adjustment over a broad range, they have relatively poor frequency stability and generate excessive phase noise due to their use of a Schmitt trigger to actuate a timing control unit.